Embodiments of the present invention concern the process of switching the power supply of an electric engine operating with alternating current between a power supply from an adjustable speed drive (ASD) and an electrical network. Embodiments of the present invention also relate to a device for controlling the power supply of an associated electric engine.
An electric engine converts electrical energy into mechanical energy. An electric engine is composed primarily of a motor.
For certain applications, speed requirements are specific to the motor of the electric engine. This is the case particularly in the field of petroleum or gas compression. In such cases, it is desirable that the power supply to the motor can be rapidly switched from an electrical network to an adjustable speed drive or vice versa.
An adjustable speed drive is especially suitable for starting the motor. Once it has been started, the motor must be powered by the electrical network, and moreover through an ASD. This requires the voltages supplied by the ASD and the network to be synchronised. This serves to prevent surges in the voltage or the current at the moment of switching, which can be detrimental to the motor of the electric engine.
It is known that the processes for starting a motor comprise two stages: an initial step in which the motor is accelerated until it reaches a required speed, followed by a second step, in which the voltage levels supplied by the adjustable speed drive and the network are synchronised.
According to the Hazel document entitled “Génération électrique intégrée aux sites industriels et bâtiments commerciaux” (Integrated Electrical Generation in Industrial Sites and Commercial Buildings), published in the Schneider Electric Technical Bulletin No. 196, dated 2000, this synchronisation is achieved by a synchronisation device composed of a synchronisation unit and a synchro-check relay. A synchro-check relay is an apparatus that measures the amplitude and the phase difference between the voltage provided by the adjustable speed drive and the network voltage. The synchro-check can verify the conformity of the amplitude and the phase measured against a synchronisation criterion.
If the synchronisation criterion is not satisfied, the synchronisation is activated. This unit sends instructions to an adjustable speed drive controller so that the latter can correct the measured difference. The instructions are generally sent in the form of electrical impulses.
When this technique is used for starting, the acceleration stage lasts as long as 10 seconds and the synchronisation stage is active for 20 seconds. Thus, the overall process takes 30 seconds. This is long for certain ASDs that supply the motor. In certain cases, this period is long enough that the heating of the adjustable speed drive requires a cooling by water or oversizing.
There is therefore a need for a method of switching the power supply of an electric engine between a power supply by an adjustable speed drive and an electrical network, which is activated more quickly.